Single contact, single absorption system for producing sulfuric acid with high conversion efficiency

ABSTRACT

Commercial production of sulfuric acid is almost entirely accomplished nowadays using the contact process. And the trend is to increase conversion efficiency and reduce emissions of unconverted sulfur dioxide. By using a special combination of contact catalyst beds, a single contact single absorption (SCSA) system can be engineered to achieve the conversion and emission capabilities of conventional double contact double absorption systems. Thus, the complexity and cost of incorporating a second absorption tower and associated heat exchanger in the system can be omitted. In the SCSA system, the initial catalyst bed or beds comprise vanadium oxide catalyst and the last catalyst bed or beds comprise platinum catalyst operating at a much lower temperature than the initial beds.

TECHNICAL FIELD

The present invention pertains to systems for oxidizing sulphur dioxideaccording to the contact process and for producing sulphuric acidthereafter. In particular, it pertains to single contact singleabsorption systems having high conversion efficiency and low emissions.

BACKGROUND

Sulfuric acid is one of the most produced commodity chemicals in theworld and is widely used in the chemical industry and commercialproducts. Generally, production methods involve converting sulphurdioxide first to sulphur trioxide which is then later converted tosulphuric acid. In 1831, P. Phillips developed the contact process whichis used to produce most of today's supply of sulphuric acid.

The basics of the contact process involve obtaining a supply of sulphurdioxide (e.g. commonly obtained by burning sulphur or by roastingsulphide minerals) and then oxidizing the sulphur dioxide with oxygen inthe presence of a catalyst (typically vanadium oxide) to accelerate thereaction in order to produce sulphur trioxide. The reaction isreversible and exothermic and it is important to appropriately controlthe temperature of the gases over the catalyst in order to achieve thedesired conversion without damaging the contact apparatus whichcomprises the catalyst.

Then, the produced sulphur trioxide is absorbed into a concentratedsulphuric acid solution to form a higher strength sulfuric acidsolution, which is then diluted with water to return the higher strengthsolution to the original concentration. This avoids the consequences ofdirectly dissolving sulphur trioxide into water which is a highlyexothermic reaction.

While the fundamentals of the contact process are relatively simple, itis desirable to maximize the conversion of sulfur dioxide into sulphuricacid and to minimize the emissions of unconverted sulfur dioxide. Thus,modern plants for producing sulphuric acid often involve multiplecontact stages and absorption stages to improve conversion andabsorption. Further, the plants often involve complex heat exchangerarrangements to improve energy efficiency.

While single contact single absorption (SCSA) systems remain in use,more complex double contact double absorption (DCDA) systems are oftenemployed in order to achieve the ever increasing requirements for higherconversion efficiency and reduced emissions. In a DCDA system, processgases are subjected to two contact and absorption stages in series,(i.e. a first catalytic conversion and subsequent absorption stepfollowed by a second catalytic conversion and absorption step). Detailsregarding the conventional options available and preferences forsulphuric acid production and the contact process are well known and canbe found for instance in “Handbook of Sulfuric Acid Manufacturing”,Douglas Louie, ISBN 0-9738992-0-4, 2005, published by DKL Engineering,Inc., Ontario, Canada.

Platinum catalyst was historically used up to the early 1900s in systemsfor producing sulfuric acid by the contact process but had certaintechnical, availability, and economic disadvantages. The platinumcatalyst could be poisoned and suffer a loss in activity by the presenceof arsenic impurities from roasting sulphide minerals. Over a centuryago, the Mannheim process was developed to overcome these problems. Inthis process, a first conversion stage uses ferric oxide catalystfollowed by a SO₃ absorption, and then a second conversion stage usesplatinum catalyst and a final SO₃ absorption. On the economic sidehowever, platinum was and still is relatively rare and expensive.

Platinum was essentially replaced by more economic vanadium oxidecatalysts decades ago. And these vanadium oxide catalysts remain as thepredominant catalyst choice for the commercial contact process. However,substantial research has been performed towards finding improvedcatalysts or combinations of catalysts in order to achieve betterconversion, reduce cost, and so on.

For instance, U.S. Pat. No. 5,175,136 discloses a process for themanufacture of sulfuric acid in which a gas stream comprising sulfurdioxide and oxygen is passed through a plurality of preliminarycontacting stages, in each of which the gas is contacted with amonolithic catalyst comprising a platinum active phase, therebyconverting a substantial fraction of the sulfur dioxide in the gasstream to sulfur trioxide. The gas stream leaving one of the pluralityof preliminary contacting stages is contacted with sulfuric acid in anabsorption zone to remove sulfur trioxide from the stream by absorptionin the sulfuric acid. After having passed through the plurality ofpreliminary stages and the absorption zone, the gas stream is passedthrough a final contacting stage in which it is contacted with aparticulate catalyst comprising vanadium and cesium, therebysubstantially converting residual sulfur dioxide in the gas to sulfurtrioxide. Platinum was not used at low temperatures and low sulfurdioxide concentrations.

As another example, US2008/0226540 discloses certain ruthenium oxidecatalysts that are used in final contact stage for conversion of SO₂ toSO₃ in multiple stage catalytic converters used in sulfuric acidmanufacture. The ruthenium oxide catalysts here provide improved lowtemperature conversion.

In yet another example, improved emissions using specific combinationsof cesium-promoted and conventional vanadium pentoxide catalysts wasdisclosed in “Optimisation of Anglo Platinum's ACP Acid Plant CatalyticConverter”, M. Sichone, The Southern African Institute of Mining andMetallurgy, Sulphur and Sulphuric Acid Conference 2009.

The contact process can be carried out under adiabatic or isothermalconditions. Most commonly, commercial sulphuric acid plants operateunder adiabatic conditions, although isothermal operation can offerpotential advantages in principle. GB1504725 for instance discloses aprocess which may be isothermal for the manufacture of sulfur trioxide,which comprises contacting technically pure sulfur dioxide and oxygen ina tubular heat exchanger in the presence of a suitable catalyst. Nearlypure SO₃ can generally be obtained. A catalyst based on vanadiumpentoxide is particularly suitable for this process. However, a platinumcatalyst and an iron oxide catalyst may also be used. A suitableoperating temperature for a V₂O₅ catalyst is from 420 to 630° C., for aFe₂O₃ catalyst from 500 to 780° C. and for a platinum catalyst from 400to 750° C. If a platinum catalyst is used, those surfaces of the heatexchanger bounding the reaction zone may for example be coated withplatinum, a platinum network may be hung into the reaction zone, forexample parallel to the axis of the heat exchanger tubes or the reactionzone may be filled with spirally rolled nets. The heat exchangerreaction tube is preferably filled with the catalyst in lump form.Oxidation and heat development occur inside this heat exchanger tube,the heat of the reaction is conducted off directly via the tube wallsand consequently the process is isothermal.

Another approach for isothermal or “pseudoisothermal” operation wassuggested in U.S. Pat. No. 7,871,593 which discloses a process for thecontinuous catalytic complete or partial oxidation of a starting gascontaining from 0.1 to 66% by volume of sulphur dioxide plus oxygen, inwhich the catalyst is kept active by means of pseudoisothermal processconditions with introduction or removal of energy. Apparatus for thecontinuous catalytic complete or partial oxidation of a starting gascontaining sulphur dioxide and oxygen having at least one tube contactapparatus is disclosed in the form of an upright heat exchanger composedof at least one double-walled tube whose catalyst-filled inner tubeforms a reaction tube. Heat is transferred in cocurrent fashion aroundthe reaction tube using an externally supplied cooling medium (such asair). Objects of the invention were to make possible the inexpensivepreparation of sulphuric acid for concentrated starting gases havingsulphur dioxide contents of >13.5% by volume and also to provide aneconomically ecological process for sulphur dioxide-containing off gasesfrom various chemical processes.

Notwithstanding the work done to date in the art, there remains a needfor yet further improvements in conversion and energy efficiency, andreductions in emissions and cost in the industrial production ofsulphuric acid. The present invention addresses this need and providesother benefits as disclosed below.

SUMMARY

In the present invention, a single contact, single absorption (SCSA)system for oxidizing sulfur dioxide to produce sulfuric acid isdisclosed which can provide the same conversion efficiencies and/orsulfur dioxide emissions of conventional double contact doubleabsorption (DCDA) systems. Thus the additional complexity and equipmentrequirements of DCDA systems are avoided, and particularly therequirement for a second absorption tower and associated heat exchangersubsystem can be omitted. Consequently, the overall system can besimplified, and energy and capital cost benefits can be obtained.

Specifically, the SCSA system for oxidizing sulfur dioxide to producesulfuric acid comprises an inlet for a gas supply comprising sulfurdioxide and oxygen, a series of contact catalyst beds, an absorptiontower, and a sulfur trioxide heat exchanger. The SCSA system is uniquein that the series of contact catalyst beds comprises one or morevanadium oxide catalyst beds fluidly connected in series to the gassupply inlet and one or more platinum catalyst beds fluidly connected inseries to (or alternatively replacing) the last vanadium oxide catalystbed in the series of vanadium oxide catalyst beds. Each of the contactcatalyst beds in the system comprises an inlet and outlet. The systemadditionally comprises a platinum catalyst bed heat exchanger with oneside connected between the outlet of the last vanadium oxide catalystbed in the series and the inlet of the first platinum catalyst bed inthe series. The system may comprise additional heat exchangers, such asa vanadium oxide catalyst bed heat exchanger or exchangers in which oneside is connected between the outlet of one of the vanadium oxidecatalyst beds in the series and the inlet of the next one of thevanadium oxide catalyst beds in the series. The absorption tower has aninlet that is fluidly connected to the outlet of the last platinumcatalyst bed in the series. And the sulfur trioxide heat exchanger hasone side connected between the outlet of the last platinum catalyst bedin the series and the absorption tower inlet.

Because platinum catalyst is catalytically active at lower temperaturesthan conventional vanadium oxide catalyst, the final platinum basedcontact catalyst bed or beds in the series can be operated at lowertemperatures and thereby obtain a more favorable final stage conversionof sulfur dioxide from the system. However, the initial conversionstages of sulfur dioxide can still desirably be accomplished usingconventional vanadium oxide catalyst. Further, the platinum contactcatalyst bed or beds in the series are protected against poisoning fromany arsenic or other impurities in the gas supply by the initialvanadium oxide based contact catalyst bed or beds.

In one embodiment, the series of vanadium oxide catalyst beds in theSCSA system consists of three vanadium oxide catalyst beds. In anotherembodiment, the series of vanadium oxide catalyst beds can consist offour vanadium oxide catalyst beds. In yet other embodiments, a differentnumber of vanadium oxide catalyst beds may be considered. In anexemplary embodiment, the series of platinum catalyst beds in the SCSAsystem consists of just one platinum catalyst bed. However, in otherembodiments more than one platinum catalyst bed may be used. An optionalscrubber which is fluidly connected to the outlet of the absorptiontower may also be used in any of these systems.

The platinum catalyst can be provided in the form of pellets.Alternatively, the platinum catalyst can be coated onto a surfaceselected from the group consisting mesh, monoliths, and tube inserts.The contact converter comprising the platinum catalyst bed can be in theform of an adiabatic, an isothermal, or a quasi-isothermal (e.g.pseudoisothermal) converter. The construction of the converter can betubular or plate type and can essentially be constructed as a heatexchanger with catalyst incorporated therein.

Improved conversion efficiency is obtained in the aforementioned SCSAsystem by directing a gas stream comprising sulfur dioxide and oxygen tothe gas supply inlet at a vanadium oxide catalyst activationtemperature, passing the gas stream through the series of vanadium oxidecatalyst beds thereby converting sulfur dioxide in the gas stream tosulfur trioxide, cooling the gas stream from the outlet of the lastvanadium oxide catalyst bed in the series of vanadium oxide catalystbeds in the platinum catalyst bed heat exchanger to a platinum catalystactivation temperature in the range from about 250 to 350° C., passingthe gas stream through the series of platinum catalyst beds therebyconverting sulfur dioxide in the gas stream to sulfur trioxide, andabsorbing the sulfur trioxide in the gas stream into water in theabsorption tower, thereby producing sulfuric acid.

The method is suitable for use with gas supplies in which theconcentration of sulfur dioxide is greater than or equal to 3%, andparticularly greater than or equal to 11%. Further, the method issuitable for use with gas supplies comprising arsenic impurity, e.g.metallurgical gas supplies obtained from roasting sulphide minerals.

Significantly, the platinum catalyst activation temperature is lowerthan the vanadium oxide catalyst activation temperature. As mentioned,the former is in the range from about 250 to 350° C., and preferably inthe range from about 275 to 325° C. The latter is in the conventionalrange from about 385 to 425° C. Generally, the temperature of the gasstream from the outlet of the last vanadium oxide catalyst bed in theseries is in the range from about 400 to 450° C. Thus, the gas stream iscooled by more than 100° C. in the platinum catalyst bed heat exchanger.

Using the method of the invention, conversion efficiencies and sulfurdioxide emissions typical of a conventional DCDA system can be obtainedin a simpler SCSA system. For instance in the present SCSA system,greater than 99.7% of the sulfur dioxide in the gas stream can beconverted to sulfur trioxide and the gas stream can comprise less than450 ppmv after passing the gas stream through the series of platinumcatalyst beds. Advantageously, the method can comprise recoveringgreater than or equal to 95% of the relatively expensive platinumcatalyst in the single contact, single absorption system at the end ofits life cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a single contact single absorption systemfor producing sulphuric acid. A prior art SCSA system differs from thatof the present invention in the types of catalyst used in the series ofcontact catalyst beds.

DETAILED DESCRIPTION

Unless the context requires otherwise, throughout this specification andclaims, the words “comprise”, “comprising” and the like are to beconstrued in an open, inclusive sense. The words “a”, “an”, and the likeare to be considered as meaning at least one and are not limited to justone.

In a numerical context, the word “about” is to be construed as meaningplus or minus 10%.

The term “catalyst bed” has been used herein to refer to a mass orcollection of catalyst. It can be in the form of a pile, layer, coating,or any other arrangement of catalyst mass. It is not intended to limitthe form or manner in which the catalyst has been compiled.

Further, “platinum catalyst” and “platinum catalyst bed” refer herein tocatalyst or a catalyst bed comprising platinum which can be in the formof platinum metal, platinum oxides or alloys, and which can alsocomprise metal oxide promoters or the like.

The heat exchangers referred to herein are devices for exchanging heatbetween two different fluids and are considered to have two sidesseparated by a heat exchange surface. Each of the different fluids isdirected to a different side in the heat exchanger and heat is exchangedbetween the two through the heat exchange surface.

FIG. 1 shows a schematic of a SCSA system for oxidizing sulfur dioxideto produce sulphuric acid. The general construction and configuration ofa SCSA system of the invention is similar to that of a prior art SCSAsystem, and thus the schematic of FIG. 1 is common to both. Where theydiffer is in the types of and arrangement of the contact catalyst bedsused.

As shown, SCSA system 5 includes contact converter 12 which comprisesfour contact catalyst beds 1, 2, 3, 4 in series. Each contact catalystbed has an inlet, namely 1 a, 2 a, 3 a, 4 a and an outlet, namely 1 b, 2b, 3 b, 4 b. System 5 also comprises inlet 6 for a gas supply comprisingan appropriate mixture of sulfur dioxide and oxygen (e.g. 12% SO₂, 12%O₂ in N₂). And system 5 comprises absorption tower 8 and sulfur trioxideheat exchanger 7. Outlet 4 b of the last contact catalyst bed 4 in theseries is fluidly connected to one side of sulfur trioxide heatexchanger 7, which in turn is fluidly connected to the inlet ofabsorption tower 8.

In the embodiment of FIG. 1, system 5 also comprises a heat exchangerbetween each pair of contact catalyst beds in the series of beds.Specifically, catalyst bed heat exchanger 9 is connected between outlet1 b of contact catalyst bed 1 and inlet 2 a of contact catalyst bed 2.Catalyst bed heat exchanger 10 is connected between outlet 2 b ofcontact catalyst bed 2 and inlet 3 a of contact catalyst bed 3. Andcatalyst bed heat exchanger 11 is connected between outlet 3 b ofcontact catalyst bed 3 and inlet 4 a of contact catalyst bed 4.

In a typical commercial SCSA system of the prior art, contact catalystbeds 1, 2, 3, and 4 are all vanadium oxide based catalyst beds. In theinvention however, one or more of the final contact catalyst beds in theseries are instead platinum based catalyst beds. For instance, in anexemplary embodiment of the invention, the last contact catalyst bed 4in the series is a platinum based catalyst bed, while the others remainvanadium oxide based.

Unlike conventional vanadium oxide catalyst, platinum will initiatecatalyst activity and maintain a practical rate of conversion at lowertemperatures (e.g. at or below 350° C.) in the last contact catalystbed. Thus, improved efficiencies can be obtained. Platinum generallyinitiates catalytic activity at lower temperatures than conventionalbeds. Further however, the reaction rate of platinum is roughlyindependent of the oxygen concentration and thus conversion can stillproceed at a practical level even though the oxygen concentration in thelast catalyst bed is much lower than in the initial beds. (On the otherhand, the reaction rate of conventional vanadium oxide catalyst isroughly proportional to the oxygen concentration and thus conversionproceeds much slower in a last vanadium oxide bed.) Further still, thereaction rate of platinum roughly varies as the inverse of the squareroot of the sulfur trioxide concentration and thus again conversion canstill proceed at a practical level even though the sulfur trioxideconcentration in the last catalyst bed is significantly higher than inthe initial beds. (On the other hand, the reaction rate of conventionalvanadium oxide catalyst varies roughly as the inverse of the sulfurtrioxide concentration and thus again conversion proceeds much slower ina last vanadium oxide bed.)

The SCSA systems of the invention thus differ from those of the priorart in terms of the operating parameters employed for the last platinumcatalyst bed or beds in the series. Specifically, the platinum catalystis operated at substantially lower temperatures than vanadium oxidecatalyst is operated at. An advantage of platinum catalyst is that it iscatalytically active to much lower temperatures than are vanadium oxidecatalysts. The equilibrium characteristics for sulfur dioxide conversionfavour more complete conversion at temperatures below the loweroperating limit of vanadium oxide catalyst. Thus, operating the finalplatinum based contact catalyst bed at temperatures much below theconventional limit of about 385° C. can provide for improved systemconversion and thus emissions. The known disadvantage of poisoning ofthe platinum catalyst by arsenic is overcome by retaining vanadium oxidecatalyst in the initial beds in the system. These initial vanadium oxidebased contact catalyst beds protect the downstream platinum contactcatalyst bed from poisoning from any arsenic or other relevantimpurities. Although additional cost is expected with the use ofplatinum in place of vanadium oxide, the extra cost is not assubstantial if only replacing a final contact catalyst bed with platinumwhen compared to replacing all the contact catalyst beds with platinum.And importantly, the inventors have discovered that using platinumcatalyst in a select final bed or beds can allow for a SCSA system toprovide product with conversion efficiency and emissions comparable orbetter to more complex and expensive SCSA systems.

In the exemplary embodiment of the invention then, contact catalyst beds1, 2, and 3 are vanadium oxide contact catalyst beds while contactcatalyst bed 4 is a platinum contact catalyst bed. A gas supplycomprising SO₂ and O₂ is obtained from a suitable source (e.g. roastingor sulfur burning source) at a temperature in the range from about 100to 420° C. A wide range of SO₂ concentrations can be processed using theinventive method (e.g. [SO₂] greater than or equal to 3%).Advantageously, gas supplies comprising greater than or equal to 11% SO₂can be processed.

The gas supply is then heated using an appropriate heat exchanger (notshown in FIG. 1) to a vanadium oxide catalyst activation temperature inthe range from about 385 to 425° C. The gas supply then is streamed intosystem 5 at inlet 6 and is directed to inlet 1 a of initial vanadiumoxide contact catalyst bed 1. A fraction of the sulfur dioxide isexothermically converted to sulfur trioxide within and the gas streamthen exits at outlet 1 b at a temperature in the range from about 450 to630° C. (depending on gas concentration). From there, the gas stream isdirected to vanadium oxide catalyst bed heat exchanger 9 where it iscooled again to an appropriate vanadium oxide catalyst activationtemperature (about 425 to 450° C.). The gas stream is then directed toinlet 2 a of the next vanadium oxide contact catalyst bed 2 in theseries.

In a like manner, another fraction of the sulfur dioxide isexothermically converted to sulfur trioxide within bed 2. The gas streamexits at outlet 2 b at elevated temperature and is directed to vanadiumoxide catalyst bed heat exchanger 10, where it is cooled again to thevanadium oxide catalyst activation temperature. The gas stream is thendirected to inlet 3 a of the next vanadium oxide contact catalyst bed 3in the series. Again, another fraction of the sulfur dioxide isexothermically converted to sulfur trioxide. The gas stream exits atoutlet 3 b at elevated temperature (in the range from about 400 to 450°C.), and this time is directed to platinum catalyst bed heat exchanger11 where it is cooled to the lower platinum catalyst activationtemperature in the range from about 250 to 350° C.

The gas stream is then directed to inlet 4 a of platinum contactcatalyst bed 4, in which sulfur dioxide is converted to sulfur trioxidewith very high conversion efficiency (e.g. >99.7%). System 5 thusprovides conversion efficiency and emissions comparable to or betterthan conventional DCDA systems.

The gas stream now contains sulfur trioxide and almost no sulfurdioxide. After exiting outlet 4 b, the gas stream is cooled in sulfurtrioxide heat exchanger 7 and is then directed to the inlet ofabsorption tower 8. Therein, the sulfur trioxide is absorbed in water toproduce sulfuric acid. The remaining gas is then typically vented from astack (not shown in FIG. 1). The sulfur dioxide content in the ventedgas is very low (e.g. 450 ppmv or lower) and can be comparable or betterto the emissions from conventional DCDA systems.

It is expected that the catalyst in such a SCSA system would not needreplacing for a significant time (e.g. 5 years or so). And unlikeconventional vanadium oxide catalyst, most of the relatively expensiveplatinum in the catalyst can be recovered at the end of system life(e.g. about 95% recovery).

While the preceding description represents a desirable exemplaryembodiment of the invention, it will readily be apparent to those in theart that other configurations employing the above invention arepossible. For instance, systems with more than three initial vanadiumoxide based contact catalyst beds in series may be contemplated, as cansystems with more than one final platinum based contact catalyst bed inseries. Further, a scrubber may optionally be employed after theabsorption tower.

With regards to contact converter 12, it may be a single unit comprisingall the contact catalyst beds in an appropriate arrangement.Alternatively, it may comprise two or more component converters with thecontact catalyst beds split up appropriately between them. For instance,the vanadium oxide contact catalyst beds may all be contained in asingle converter, while the platinum contact catalyst bed may becontained in a separate adiabatic, isothermal, or quasi-isothermalconverter.

A variety of designs may be considered for contact converter 12 and/orthe component converters within. Particularly, any conventionalarrangement may be employed for the vanadium oxide contact catalystbeds. With regards to the platinum contact catalyst bed, it may beprovided in a variety of ways. For instance, platinum catalyst may beprovided in pellet form or as coatings on an appropriate surface (e.g.mesh, monoliths, or tube inserts or plates for heat exchanger-likeconstructions).

As mentioned above, the invention allows sulfur trioxide (and from thatsulfuric acid) to be produced in a SCSA system with a conversionefficiency and emissions similar to that obtained from a DCDA system.The extra absorption tower, any associated heat exchanger, piping, andcontrols that typically appear in a DCDA are no longer needed to obtainsimilar desirable results.

It will thus be apparent to those in the art that the invention may beused to construct new SCSA systems as well as to retrofit existingconventional SCSA systems such that conversion efficiency and emissionssimilar to a DCDA system is obtained. For instance, a suitable retrofitof a conventional SCSA system can merely involve replacement of a fourthconventional vanadium oxide based, contact catalyst bed with a platinumbased, contact catalyst bed, and modification of heat exchangers andsystem controls such that the operating temperatures are changedappropriately.

Further, it will be apparent to those in the art that similar benefitsin conversion efficiency and emissions can be expected when employing aplatinum contact catalyst bed or beds after the intermediate absorptiontower in a DCDA system.

The following Example has been included to illustrate certain aspects ofthe invention but should not be construed as limiting in any way.

Examples

Calculations were made to determine the expected performance from anotherwise conventional SCSA system that had been retrofitted to includea final platinum catalyst based bed according to the invention.

The conventional SCSA system was assumed to have a capacity of about2000 metric tons per day. A metallurgical supply of gas was assumed asthe feed gas and contained 11.5% SO₂ with a 1:1 O₂/SO₂ ratio. This feedgas was supplied at about 1.6×10⁵ Nm³/hr. The SCSA system was furtherassumed to comprise four conventional vanadium oxide based catalyst bedsin series. The feed gas was supplied to the first bed at 420° C. andabout 0.26 barg, and exited the last bed at about 446° C. and about 0.16barg. Such a conventional system can produce sulfur trioxide productwith 95% conversion efficiency.

For calculation purposes, this SCSA system was then considered to havebeen retrofitted to include an additional (i.e. 5^(th)) catalyst bed inseries with the 4^(th) conventional bed. The additional bed was assumedto contain standard pellet catalyst comprising about 350 kg of platinumand also a certain amount of metal oxide promoter. In addition, thesystem also included an additional heat exchanger between the 4^(th)conventional bed and the additional platinum based catalyst bed (similarto platinum catalyst bed heat exchanger 11 in FIG. 1).

The same feed gas supply and operating conditions for the conventionalbeds was assumed. After being cooled by the additional heat exchanger,gas entered the 5^(th) platinum based catalyst bed at 300° C. and about0.13 barg, and exited the last bed at about 317° C. and about 0.10 barg.Under these conditions, it is expected that a 98.2% approach toequilibrium can be attained. The retrofitted system is then expected toproduce sulfur trioxide product with 99.85% conversion efficiency andwith sulfur dioxide emissions less than 220 ppmv. It is expected thatthe catalyst in this system would not need replacing for about 5 years,at which point about 95% of the platinum in the catalyst could berecovered. Hence the system can provide desirable conversionefficiencies and emissions and further is viable economically.

All of the above U.S. patents, U.S. patent applications, foreignpatents, foreign patent applications and non-patent publicationsreferred to in this specification, are incorporated herein by referencein their entirety.

While particular elements, embodiments and applications of the presentinvention have been shown and described, it will be understood, ofcourse, that the invention is not limited thereto since modificationsmay be made by those skilled in the art without departing from thespirit and scope of the present disclosure, particularly in light of theforegoing teachings. Such modifications are to be considered within thepurview and scope of the claims appended hereto.

1. A single contact, single absorption system for oxidizing sulfurdioxide to produce sulfuric acid comprising: an inlet for a gas supplycomprising sulfur dioxide and oxygen; a series of contact catalyst bedscomprising one or more vanadium oxide catalyst beds fluidly connected inseries to the gas supply inlet and one or more platinum catalyst bedsfluidly connected in series to the last vanadium oxide catalyst bed inthe series of vanadium oxide catalyst beds, wherein each of the contactcatalyst beds comprises an inlet and outlet; a platinum catalyst bedheat exchanger with one side connected between the outlet of the lastvanadium oxide catalyst bed in the series and the inlet of the firstplatinum catalyst bed in the series; an absorption tower with an inletfluidly connected to the outlet of the last platinum catalyst bed in theseries; and a sulfur trioxide heat exchanger with one side connectedbetween the outlet of the last platinum catalyst bed in the series andthe absorption tower inlet.
 2. The single contact, single absorptionsystem of claim 1 wherein the series of vanadium oxide catalyst bedsconsists of three vanadium oxide catalyst beds.
 3. The single contact,single absorption system of claim 1 wherein the series of vanadium oxidecatalyst beds consists of four vanadium oxide catalyst beds.
 4. Thesingle contact, single absorption system of claim 1 wherein the seriesof platinum catalyst beds consists of one platinum catalyst bed.
 5. Thesingle contact, single absorption system of claim 1 wherein the platinumcatalyst is in the form of pellets.
 6. The single contact, singleabsorption system of claim 1 wherein the platinum catalyst is coatedonto a surface selected from the group consisting mesh, monoliths, andtube inserts.
 7. The single contact, single absorption system of claim 1comprising a vanadium oxide catalyst bed heat exchanger with one sideconnected between the outlet of one of the vanadium oxide catalyst bedsin the series and the inlet of the next one of the vanadium oxidecatalyst beds in the series.
 8. The single contact, single absorptionsystem of claim 1 comprising no other absorption tower.
 9. A method forimproving conversion efficiency in the production of sulfuric acid byoxidizing sulfur dioxide in a single contact, single absorption system,the method comprising: providing the single contact, single absorptionsystem of claim 1; directing a gas stream comprising sulfur dioxide andoxygen to the gas supply inlet at a vanadium oxide catalyst activationtemperature; passing the gas stream through the series of vanadium oxidecatalyst beds thereby converting sulfur dioxide in the gas stream tosulfur trioxide; cooling the gas stream from the outlet of the lastvanadium oxide catalyst bed in the series of vanadium oxide catalystbeds in the platinum catalyst bed heat exchanger to a platinum catalystactivation temperature in the range from about 250 to 350° C.; passingthe gas stream through the series of platinum catalyst beds therebyconverting sulfur dioxide in the gas stream to sulfur trioxide; andabsorbing the sulfur trioxide in the gas stream into water in theabsorption tower, thereby producing sulfuric acid.
 10. The method ofclaim 9 wherein the concentration of sulfur dioxide in the gas supply isgreater than or equal to 3%.
 11. The method of claim 10 wherein theconcentration of sulfur dioxide in the gas supply is greater than orequal to 11%.
 12. The method of claim 9 wherein the gas supply comprisesarsenic.
 13. The method of claim 12 wherein the gas supply is ametallurgical gas supply.
 14. The method of claim 9 wherein the platinumcatalyst activation temperature is in the range from about 275 to 325°C.
 15. The method of claim 9 wherein the vanadium oxide catalystactivation temperature is in the range from about 385 to 425° C.
 16. Themethod of claim 9 wherein the temperature of the gas stream from theoutlet of the last vanadium oxide catalyst bed is in the range fromabout 400 to 450° C.
 17. The method of claim 9 wherein greater than99.7% of the sulfur dioxide in the gas stream is converted to sulfurtrioxide after passing the gas stream through the series of platinumcatalyst beds.
 18. The method of claim 9 wherein the gas streamcomprises less than 450 ppmv of sulfur dioxide after passing through theseries of platinum catalyst beds.
 19. The method of claim 9 comprisingrecovering greater than or equal to 95% of the platinum in the singlecontact, single absorption system at the end of its life cycle.